The present invention relates to a method for preparing a layered perovskite compound thin film with organic ammonium layers and inorganic layers, and a layered perovskite compound thin film with organic ammonium layers and inorganic layers prepared thereby in which the organic ammonium molecule contains a functional chromophore.
It is known that a layered perovskite compound has a superlattice structure in which an organic layer and an inorganic layer are alternately stacked, and exhibits various interesting properties useful for low-dimensional semiconductors, magnetic bodies, luminous bodies or the like, depending on the type of inorganic halide layer. In addition, it is suggested that the layered perovskite compound has the potential of providing new functional materials capable of bringing out combined functionalities of organic layers and inorganic layers (M. Era, K. Maeda, and T. Tsutsui, Chemistry Letters, 1235-1236, 1997).
One of known layered perovskite compounds has a superlattice structure which is represented by the general formula A2MX4 where A is an organic ammonium molecule, M is a group 4 element or transition metal and X is halogen (Cl, Br, I), and in which a layer of organic ammonium molecule xe2x80x9cAxe2x80x9d and a layer of inorganic halide xe2x80x9cMX4xe2x80x9d are alternately stacked (see FIG. 10 showing a schematic diagram thereof). Further, another known layered perovskite compound has a superlattice structure which is represented by the general formula A2MX4, where A is organic ammonium ions, M is divalent rare earth metal cations and X is halogen (Cl, Br, I), and in which a layer of organic ammonium molecule xe2x80x9cAxe2x80x9d and a layer of inorganic halide xe2x80x9cMX4xe2x80x9d are alternately stacked (Japanese Patent Laid-Open Publication No. Hei 10-316685).
A conventional method for preparing a thin film from such layered perovskite compounds includes one method (a casting method or spin-coating method) in which a crystal of a layered perovskite compound or a mixture of an organic amine hydrohalic acid salt and an inorganic halide MX2 is dissolved in an organic solvent and then a thin film is formed from the resulting solution, and another method (a two-source deposition method) in which an organic amine hydrohalic acid salt and an inorganic halide MX2 are deposited simultaneously in vacuo.
Heretofore, as a method for depositing monomolecular films, there has been known the Langmuir-Blodgett method (LB method) used for manufacturing organic thin films. Japanese Patent Laid-Open Publication No. Hei 11-140656 disclosed a method for manufacturing an inorganic thin film in which monomolecular films are accumulated with controlling the composition and arrangement of metal elements by using the Langmuir-Blodgett method to form an organic thin film, and then the organic component of the organic thin film is decomposed and removed to form an inorganic thin film. However, such a method has not been successfully used for preparing layered perovskite compound thin films.
As shown in FIG. 9, the Langmuir-Blodgett method is a technique for preparing a thin film having a film thickness and structure controlled at the level of a monomolecular layer by forming a monomolecular film of amphipathic molecules on the water and transferring it onto a substrate (see xe2x80x9cPhysics and Chemistry Dictionary-5th Editionxe2x80x9d published by Iwanami Shoten).
(Problem to be Solved by the Invention)
For using the above layered perovskite compound with organic ammonium layers and inorganic layers as device materials, it is a critical task to control a film thickness and structure stringently at the level of a monomolecular layer. Further, if such layered perovskite compounds can be combinedly formed at the level of a monomolecular layer, it would open the way to develop various materials having higher performance. However, the conventional techniques for preparing layered perovskite compounds have not been able to prepare such a thin film having a film thickness and structure controlled at the level of a monomolecular layer.
Therefore, it is an object of the present invention to provide a novel method for preparing a layered perovskite compound thin film with organic ammonium layers and inorganic layers which has a film thickness and structure controlled at the level of a monomolecular layer, and to provide a layered perovskite compound thin film with organic ammonium layers and inorganic layers which has a novel function useful for devices, through this method.
(Means for Solving the Problem)
The inventor has found a method for forming a monomolecular film of an organic-inorganic layered perovskite compound on the surface of a water solution. Based on this method and applying the Langmuir-Blodgett method, the inventor has made it possible to prepare a layered perovskite organic-inorganic superlattice material having a film thickness and structure controlled at the level of a monomolecular layer. Further, in combination with this method, the inventor has achieved to introduce a chromophore into the organic layers so as to allow the layered perovskite organic-inorganic superlattice material to be applied to electroluminescence (EL) devices.
More specifically, according to a first aspect of the present invention, there is provided a method for preparing a layered perovskite compound thin film with organic ammonium layers and inorganic layers which has a superlattice structure represented by the general formula A2MX4 where xe2x80x9cAxe2x80x9d is an organic ammonium molecule, xe2x80x9cMxe2x80x9d is a group 4 element or transition metal and xe2x80x9cXxe2x80x9d is halogen (Cl, Br, I), and in which a layer of organic ammonium molecule xe2x80x9cAxe2x80x9d and a layer of inorganic halide xe2x80x9cMX4xe2x80x9d are alternately stacked, characterized in that an organic solvent solution having dissolved therein an organic ammonium molecule or a mixture of an organic ammonium molecule and an inorganic halide is dropped onto to the surface of a water solution having dissolved therein a water-soluble organic ammonium molecule and an inorganic halide to form a monomolecular film of an organic amine hydrohalic acid salt having a long-chain alkyl group and simultaneously to react the formed monomolecular film with the inorganic halide in the water solution so as to form a monomolecular film of a layered perovskite with organic ammonium layers and inorganic layers on the surface of the water solution, and then the formed monomolecular film is transferred onto a substrate by using the Langmuir-Blodgett method.
The group 4 element herein is Ge, Sn or Pb, and the transition metal is Cr, Mn, Fe, Co, Ni, Cu, Zn or Cd. The inorganic halide is a compound represented by the chemical formula MX2 where M is a group 4 element or transition metal and X is halogen (Cl, Br, I). The substrate for the monomolecular film to be transferred thereon may be selected from the group consisting of a silicon substrate, a metal plate, a glass plate, a fused quartz substrate, and an inorganic compound single crystal substrate.
In the method according to the first aspect of the present invention, the organic ammonium molecule to be dissolved in the water solution may be a water-soluble alkylamine hydrohalic acid salt CnH2n+1NH3X, and the organic ammonium molecule to be dissolved in the organic solvent solution may contain a non-water-soluble alkylamine hydrohalic acid salt CnH2n+1NH3X. The water-soluble alkylamine hydrohalic acid salt CnH2n+1NH3X can facilitate the formation of the layered perovskite monomolecular film on the surface of the water solution.
Further, the non-water-soluble alkylamine hydrohalic acid salt CnH2n+1NH3X to be dissolved in the organic solvent solution may be a dococylamine hydrohalic acid salt C22H45NH3X. The dococylamine hydrohalic acid salt C22H45NH3X having a long-chain alkyl group allows a stable monomolecular film to be formed on the water solution.
In the method according to the first aspect of the present invention, the organic solvent may be chloroform or a mixture of chloroform and dimethylformamide. The dimethylformamide added to the solvent allows the hardly-soluble inorganic halide to be sufficiently dissolved in the solvent solution.
Further, in the method according to the first aspect of the present invention, the organic ammonium molecule xe2x80x9cAxe2x80x9d may be an organic ammonium containing a functional chromophore. The organic ammonium containing the functional chromophore allows the layered perovskite compound thin film with organic ammonium layers and inorganic layers to be provided with the organic layers having a useful function.
In this case, the organic ammonium molecule to be dissolved in the organic solvent may be an organic ammonium containing a non-water-soluble functional chromophore. The chromophore devoid of any electrically insulative alkyl chain allows the layered perovskite compound thin film to have high conductivity. This advantageously provides an electronic function suitable for EL devices and other electronic devices.
Further, the functional chromophore may be azobenzene chromophore or salicyliden aniline chromophore. The azobenzene chromophore or salicyliden aniline chromophore exhibiting a photochromic reaction allows the property of the layered perovskite to be optically controlled.
According to a second aspect of the present invention, there is provided a method for a layered perovskite compound thin film with organic ammonium layers and inorganic layers, characterized in that a plurality of layered perovskite compound thin films are prepared by the method according to the first aspect of the present invention, wherein at least two of the layered perovskite compound thin films are different in at least either one of the kinds of the inorganic layer and organic ammonium layer, and wherein the formed layered perovskite compound thin films are combinedly stacked in increments of a monomolecular layer. There are various layered perovskites capable of exhibiting an optical, magnetic or semiconductive characteristic. Thus, according to this method, a thin film having a new functionality can be prepared by combining such layered perovskites at the level of a monomolecular layer.
According to a third aspect of the present invention, there is provided a layered perovskite compound thin film with organic ammonium layers and inorganic layers prepared by the method according to either one of the first and second aspects of the present invention, which has a superlattice structure represented by the general formula A2MX4 where xe2x80x9cAxe2x80x9d is an organic ammonium molecule, xe2x80x9cMxe2x80x9d is a group 4 element or transition metal and xe2x80x9cXxe2x80x9d is halogen (Cl, Br, I), and in which a layer of organic ammonium molecule xe2x80x9cAxe2x80x9d and a layer of inorganic halide xe2x80x9cMX4xe2x80x9d are superimposed alternately, wherein the organic ammonium molecule xe2x80x9cAxe2x80x9d is an organic ammonium containing a functional chromophore. When forming a composite layered perovskite compound thin film, the property or function of the inorganic or organic layer in each of the layered perovskites can be combined at the level of a monomolecular layer.
In the layered perovskite compound thin film with organic ammonium layers and inorganic layers according to the third aspect of the present invention, the functional chromophore may be azobenzene chromophore or salicyliden aniline chromophore. The photochromic chromophore exhibiting a certain photochromic reaction depending on lights having different wavelengths allows the property of the particular layered perovskite in the composite layered perovskite compound thin film to be optically controlled.
Further, the inorganic halide may be PbBr2 or CuBr2. The PbBr2-based layered perovskite exhibits semiconductive properties, and the CuBr2-based layered perovskite exhibits ferromagnetic properties